This invention relates to gerotor pumps which comprise a male lobed rotor meshed with an internally lobed annulus. The annulus has at least one more lobe than the rotor, and the contact lines between those two parts define boundaries of chambers which vary in volume during the rotation of the parts. The volume increases for induction of working fluid through an inlet port in approximately 180 deg. of the revolution and decreases for ejection of fluid through an outlet port in the other half of the cycle. The axes of the rotor and annulus lie in a plane which intersects the 0 deg. and 180 deg. positions.
The ports do not extend over the full 180 degrees, for a land is provided between the two ports at each of two diametric positions and the land is about equal to the circumferential dimension of one pump chamber, for the purpose of isolating the inlet from the outlet at the two positions.
In U.S. Pat. No. 3,905,727 a pump of this kind has radial grooves in the end faces of the annulus which extend from each interlobe position. Each in turn forms part of the sole flow path leading to and from the outlet and inlet port respectively.
EPA 242963 uses ports opening direct to the end faces of the chambers but one of the ports (the outlet) is divided by a web as shown in FIG. 2 of the drawings of that specification, which creates a dead spot limiting or preventing flow: in order to avoid a pressure ripple which would thus occur, the end face of the rotor has radial grooves which are of a circumferential width in excess of the width of the web, in order to allow flow from the chamber into the port around the web via the groove so that the flow may be continuous irrespective of position.
In BP A233423 the leading edges of the rotor lobes or the trailing edges of the annulus lobes are chamfered so as to connect all the chambers together via the chamfers, for the purpose of giving rapid influx to the chambers.
The inventor has discovered that noise from gerotor pumps results from pressure variations in trapped volumes. Thus if the land width exceeds the chamber width, the chamber volume will vary as it moves across the land. This can cause hammer in compression, or (which is also bad) cavitation and bubble formation in expansion. However, there are dynamic effects at work when the pump is in use which produce unexpected results when considering the geometry of the stationary parts, and this complicates the design of the land and changes the needful symmetry or asymmetry of the ports, so that it is rarely sufficient to provide a simple land of precisely the chamber width. If this is done, noise may still ensue, and pump efficiency may be lowered elsewhere.